Pretreatment for zinc and zinc alloy prior to chromating

ABSTRACT

Drying into place a covering of an aqueous solution of nickel and/or cobaltous sulfate and/or phosphate salts on a zinc or zinc alloy surface improves at least one of the blackening resistance and corrosion resistance after subsequent chromating treatment and the corrosion resistance and paint adherence after subsequent chromating treatment and painting.

TECHNICAL FIELD

The present invention relates to a method for treating zinc and zincalloy surfaces. In the description below, except where the contextrequires otherwise, the term "zinc" when used to describe surfaces to betreated is to be understood as including alloys that are predominantlyzinc. More particularly, the present invention relates to a method fortreating a surface of zinc-plated or zinc alloy-plated steel, prior to achromating treatment, to provide corrosion resistance and/or function asa paint undercoat.

BACKGROUND ART

Sacrificial anodic protection based on zinc plating or zinc alloyplating is the most effective and most economical method for thecorrosion protection of iron and steel. As a consequence, galvanizedsteel sheet accounts for 10 million tons or 10% of Japan's annual rawsteel output of 100 million tons. Galvanized steel sheet is widelyemployed for building materials, automobiles, household electricalappliances, and the like.

In sacrificial anodic protection by zinc, the two metals (zinc and ironor steel) are in contact and form an electrochemical cell, and the zinc,as the baser metal, becomes the anode and renders the iron cathodic.This inhibits corrosion of the iron or steel by preventing the anodicdissolution which would occur in the case of iron by itself due to localcell formation. Accordingly, when the zinc in contact with the iron orsteel has finally been consumed, the anticorrosion activity disappears.Therefore, preventing corrosion of the zinc layer itself (generallywhite rust corrosion) is crucial for extending the durability ofgalvanized steel material, and to this end galvanized steel is normallychromated as an undercoating treatment and then painted.

However, these two anticorrosion treatments (chromate treatment andpainting) suffer from certain problems. Thus, while the chromatetreatment of zinc-plated steel sheet or zinc alloy-plated steel sheetresults in a very substantial inhibition of white rust development, thistreatment can cause the development of black rust (known as"blackening") during storage or transport of the galvanized steel sheet.It has been observed that this phenomenon tends to occur more readily inthe case of a skin pass-rolling after zinc plating and more easily whenseveral % aluminum is present in the zinc than for ordinary galvanizedsteel sheet.

On the other hand, there has been a substantial proliferation in metalmaterials in the sector concerned with the production of coloredgalvanized steel sheet (widely employed for roofing and siding) bypainting zinc-plated steel sheet and zinc alloy-plated steel sheet,particularly in coil form. Because the available surface treatments withreactive phosphate salts have not been able to respond to thesedevelopments, coating-type chromate treatments, which can be applied tomany types of materials, tend to be used for surface treatment. However,adherence by the paint film is a normal problem here, and a problematicpaint film adherence is associated with the bending of galvanized steelsheet and particularly with the bending of ultralow-lead galvanizedsteel sheet and zinc/aluminum alloy-plated steel sheet.

Flash treatment with, e.g., Ni, Co, Fe, etc., as described, e.g., inJapanese Patent Publication Numbers 52-22,618 [22,618/77]and 52-43,171[43,171/77]and Japanese Patent Application Laid Open [Kokai orUnexamined]Number 59-177,381 [177,381/84]) is considered at present tobe a powerful countermeasure against both blackening after chromatingand low paint adherence of painted galvanized steel sheet. JapanesePatent Publication Numbers 52-22,618 and 52-43,171 teach that animproved paint adherence is obtained through a substitutional platingtreatment (executed prior to chromating) with an acidic solution (pHabout 1.5) containing, e g., Ni²⁺, Co²⁺, Fe²⁺, Fe³⁺. In the examplesprovided therein, the substitutional plating treatment is immediatelyfollowed by a water rinse.

Japanese Patent Application Laid Open Number 59-177,381 teaches thatblackening can be prevented through the treatment (executed prior tochromating) of zinc-plated or zinc alloy-plated steel sheet with asolution (pH=1-4 or 11-13.5) which contains Ni²⁺ or Co²⁺. In theexamples provided for this particular method, treatment with the Ni- orCo-containing solution is followed by rinsing with tap water. On thesubject of the activity of the Ni or Co, it is hypothesized in thisreference that black rusting is suppressed by their deposition as themetal or oxide.

Thus, as discussed above, it is already known that a pre-chromatingflash treatment of the surface of zinc plating or zinc alloy platingwith a transition metal such as Co, Fe, etc. will prevent black rustingon the chromated zinc plate or zinc alloy plate and improve the paintadherence to zinc-plated and zinc alloy-plated steel sheet.

The mechanisms by which the aforesaid Ni, Co, or Fe flash treatmentinhibit the blackening of zinc-plated or zinc alloy-plated steel sheetand increase the paint/steel sheet bonding have not yet been establishedwith certainty. However, according to the report on pages 150 to 151 ofthe Collected Summary of the 60th Scientific General Lecture Meeting ofKinzoku Hyomen Gijutsu Kyokai [The Metal Finishing Society of Japan],there is some type of interaction between the flashed metal and thechromium compound(s). This conclusion was drawn based on theobservations that a large amount of flashed metal precipitates at thegrain boundaries of the zinc crystals and that the chromium provided bythe follow-on coating-type chromate treatment is, like the flashedmetal, distributed on the grain boundaries. It is conjectured in thisreference that the chromium compound is adsorptively immobilized orfixed on the flashed metal.

Zinc black rust, like white rust, is thought to be a basic zinccarbonate as expressed by the chemical formula (ZnCO₃)_(x).[Zn(OH)₂]_(y), but it differs from white rust as a consequence of astoichiometric oxygen deficiency. Accordingly, black rust is a corrosionproduct which develops under oxygen-deficient circumstances, and, inparticular, it is thought to be formed accompanying the development ofcorrosion from the grain boundaries. Due to this, it is possible thatthe chromium compound (concentrated at the grain boundaries by theflashed metal) contributes to the inhibition of black rust developmentby inhibiting corrosion from the grain boundaries.

As to why flash treatment with a transition metal, e.g., Ni, Co, Fe,improves the paint adherence of galvanized steel sheet, it is thoughtthat the interaction between the flashed metal and chromium compound(s)serves adsorptively to fix or immobilize the chromium compound(s) on thezinc-plate surface. This strengthens the bonding forces between thezinc-plated surface of the material and the chromium compound(s), withthe result that the zinc-plate surface/chromium compound interfacialbonding strength is improved. This particular interface is considered tobe the weakest of all the interfaces between the various layers in apainted zinc surfaced object with a chromate undercoating.

As explained hereinbefore, a pre-chromating flash treatment with Ni, Co,Fe, etc., is a powerful countermeasure against both of the majorproblems associated with galvanized steel sheet (blackening andunsatisfactory paint adherence). Nevertheless, when a flash treatment isimplemented prior to chromating, it is known that black rusting is infact inhibited but that white rust tends to appear rather readily. Inaddition, although this flash treatment does improve the paint adherenceto colored galvanized steel sheet during bending, it is known that itreduces the corrosion resistance of the painted sheet and particularlythe corrosion resistance of the back-coat surface (service coat).

DESCRIPTION OF THE INVENTION

Problem to Be Solved by the Invention

A major object of this invention Was a method for treating zinc surfacesso that the development of black rust would be inhibited withoutsacrificing the white rusting resistance and the paint adherence duringbending would be improved without diminishing the corrosion resistanceof a subsequently painted sheet.

SUMMARY OF THE INVENTION

The major embodiment of the invention is a pretreatment method, to beimplemented prior to the chromating of zinc or zinc alloy, whichcomprises the application to the zinc surface of an aqueous solutionthat comprises, more preferably consists essentially of, or still morepreferably consists of, water, at least 1 selection from the sulfate andphosphate salts of Ni²⁺ and Co²⁺, and, optionally, a complexing agent tostabilize the solution, followed by drying to give a film thereon,preferably with an add-on weight of 0.5-100 milligrams per square meter(hereinafter "mg/m²). As a consequence of this pretreatment and thefollow-on execution of a chromating treatment for the purposes ofcorrosion resistance or as a paint undercoat, the major object of theinvention is achieved. For the purposes of this invention, the term"phosphate salts" is to be understood as including those salts in whichthe anions are PO₄ ³¹ 3, HPO₄ ⁻², H₂ PO₄ ⁻, P₂ O₇ ⁻², HP₂ O₇ ⁻, PO₃ ⁻,or mixtures thereof.

Additional Description of the Invention

The aqueous solution (treatment solution) employed by the presentinvention must contain at least one of the metal ions Ni²⁺ and Co²⁺,which must be supplied to the treatment solution as sulfate or phosphatesalt.

Chlorides should be avoided in the treatment solution because theyreduce the white rusting resistance, and the nitrate salts are notsatisfactory alone because they lack any effect in terms of preventingblack rust.

Otherwise, the treatment solution may optionally contain complexingagent in order to stabilize the treatment solution, and examples in thisregard are gluconic acid and heptogluconic acid and their salts such assodium gluconate and sodium heptogluconate.

The treatment method consists of the application of the treatmentsolution to the zinc or zinc alloy by any convenient coating method thatprovides adequate control of the evenness and amount of the coatingapplied, such as spray coating, immersion followed by roll squeegee,immersion followed by air knife, roll coating, and so forth. Rollcoating is usually preferred. After coating, drying is conducted withoutany intervening water rinse.

The drying conditions are not specifically restricted, and asatisfactory drying is obtained merely by removing the water in theoriginal coating. The sheet temperature preferably during drying fallswithin the range of 40°-100° C. Exceeding 100° C. does not accrue anyincrease in performance and so is economically disadvantageous.

The film weight bonded to the zinc or zinc alloy surface by theaforementioned treatment should preferably fall within the range of0.5-100 mg/m². Values less than 0.5 mg/m² do not usually result in anadequate inhibition of black rusting and cannot generally be expected toresult in an improved paint adherence when the purpose is service as apaint undercoat. On the other hand, the black rust inhibitingeffectiveness and the improvement in paint adherence both fail toimprove any further at values in excess of 100 mg/m², which makes suchvalues economically disadvantageous.

The present invention will be illustrated in greater detail belowthrough demonstrative and comparative examples. These examples areprovided simply to promote appreciation of the present invention, andthey do not restrict the present invention in any way.

EXAMPLES Examples 1-6 Comparison Examples 1-4

The sample test sheet (specified below) was subjected to an alkalinedegreasing, water rinse, drying, and then pretreatment according to thepresent invention or a comparison method. The sample test sheet (bothpretreated and non-pretreated) was then subjected to a chromatetreatment. Sample test sheets which had been subjected to thesetreatments were subsequently examined using the black rust acceleratedtesting and white rust accelerated testing described below.

The composition of the treatment solution, film weight, and the resultsfor black rust accelerated testing and white rust accelerated testingare reported in Table 1.

(1) Sample test sheet: electrogalvanized steel sheet (oiled) zincadd-on=20 g/m²

(2) Alkaline degreasing: 2% aqueous solution of weakly alkalinedegreaser (PAL-KLIN™ 342 from Nihon Parkerizing Company, Limited)temperature=60° C., spray for 30 seconds

(3) Water rinse: tap water spray, 10 seconds

(4) Drying: forced draft drying

(5) Pretreatment: The aqueous solution as reported in Table 1 wasapplied by roll coating to a wet add-on of 3 milliliters per squaremeter (hereinafter "mL/m² ").

(6) Drying: maximum sheet temperature=50° C., drying time=2 seconds

(7) Chromate treatment: aqueous solution of partially reduced chromicanhydride as described in Japanese Patent Application Laid

                  TABLE 1                                                         ______________________________________                                                                     black   white                                                                 rust    rust                                     treatment solution  film     accel-  accel-                                   composition         weight   erated  erated                                   (mole/L)            (mg/m.sup.2)                                                                           testing testing                                  ______________________________________                                        Example 1                                                                             NiSO.sub.4 :0.006                                                                              2.8     5     5                                      Example 2                                                                             CoSO.sub.4 :0.09                                                                              41.8     5     5                                      Example 3                                                                             Ni(H.sub.2 PO.sub.4).sub.2 :0.10                                                              75.8     5     5                                      Example 4                                                                             CoSO.sub.4 :0.01                                                                              19.8     5     5                                              Co(H.sub.2 PO.sub.4).sub.2 :0.02                                      Example 5                                                                             NiSO.sub.4 :0.03                                                                              13.9     5     5                                              heptogluconic acid:0.06                                               Example 6                                                                             NiSO.sub.4 :0.03                                                                              23.2     5     5                                              CoSO.sub.4 :0.02                                                      Com-    NiCl.sub.2 :0.05                                                                              19.5     4     1                                      parison                                                                       Example 1                                                                     Com-    Co(NO.sub.3).sub.2 :0.01                                                                       5.5     2     4                                      parison                                                                       Example 2                                                                     Com-    Japanese Patent Appli-                                                                        (note 2) 5     2                                      parison cation Laid Open                                                                               2.5                                                  Example 3                                                                             Number 59-177381.                                                             Example 1-d. (note 1)                                                 Com-    no pretreatment --       1     5                                      parison                                                                       Example 4                                                                     ______________________________________                                         Note 1: Rinse with tap water after the cobalt treatment.                      Note 2: Reported as the cobalt addon.                                    

Open Number 63-145785 [145,785/88](chromium weight ratio (Cr³⁺/Cr⁶⁺)=0.67, contained H₃ PO₄ and acrylic polymer emulsion with a highcompatibility and stability relative to chromic acid), roll coating,chromium add-on=45-50 mg/m² (resin film weight=180-200 mg/m²)

(8) Drying: maximum sheet temperature=80° C., drying time=5 seconds

(9) Black rust accelerated testing: Test specimens (70×150 millimeters{"mm"}) were cut from each sample test sheet. The test surfaces of thetest specimens were faced against each other to give pairs. 5-10 pairswere stacked on one another and then wrapped with vinyl-coated paper.The four corners were bolted down and a load of 70 kilograms of force("kgf") was applied using a torque wrench. The assembly was thenmaintained in a humidistat at 70° C. at a relative humidity ("RH") of80% for 240 hours. After removal, blackening of the overlaid areas wasvisually evaluated and reported on the following evaluation scale:

5: no blackening

4: extremely light greying

3: blackening over <25%

2: blackening over 25 through <50%

1: blackening over at least 50%

(10) White rust accelerated testing: A test specimen (70×150 mm) was cutfrom each sample test sheet. This test specimen was subjected to thesalt spray test specified in Japanese Industrial Standard ("JIS") Z 2371for 200 hours. The area over which white rust developed was thenvisually evaluated and reported according to the following evaluationscale:

5: no white rust

4: area of white rust development <5%

3: area of white rust development=5 to <25%

2: area of white rust development=25 to <50%

1: area of white rust development at least 50%

Examples 7-12 and Comparison Examples 5-8

The sample test sheet (specified below) was subjected to pretreatmentaccording to the present invention or a comparison method. The sampletest sheets (both pretreated and non-pretreated) were then subjected toa chromate treatment. Sample test sheets which had been subjected tothese treatments were subsequently examined using black rust acceleratedtesting and white rust accelerated testing as described below.

The composition of the treatment solution, film weight, and the resultsfor black rust accelerated testing and white rust accelerated testingare reported in Table 2.

(1) Sample test sheet: Galfan™ (zinc/5% aluminum alloy-plated steelsheet), nonoiled zinc add-on=90 g/m² elongation in skinpass-rolling=1.5%

(2) Pretreatment: immersion in the aqueous solution reported in Table 2and then air-knife squeegee to a wet pick-up of 5 mL/m²

(3 ) Drying: maximum sheet temperature=60° C., drying time=3 seconds

(4) Chromate treatment: aqueous partially reduced chromic anhydridesolution (chromium weight ratio (Cr³⁺ /Cr⁶⁺)=0.25), applied by rollcoating to give a chromium add-on of 10-15 mg/m²

                  TABLE 2                                                         ______________________________________                                                                     black   white                                                                 rust    rust                                     treatment solution  film     accel-  accel-                                   composition         weight   erated  erated                                   (mole/L)            (mg/m.sup.2)                                                                           testing testing                                  ______________________________________                                        Example 7                                                                             NiSO.sub.4 :0.05                                                                              38.7     5     5                                      Example 8                                                                             CoSO.sub.4 :0.01                                                                               7.7     5     5                                      Example 9                                                                             Co(H.sub.2 PO.sub.4).sub.2 :0.07                                                              88.5     5     5                                      Example 10                                                                            NiSO.sub.4 :0.005                                                                             10.2     5     5                                              Ni(H.sub.2 PO.sub.4).sub.2 :0.005                                     Example 11                                                                            CoSO.sub.4 :0.02                                                                              15.5     5     5                                              sodium gluconate:0.02                                                 Example 12                                                                            NiSO.sub.4 :0.005                                                                             11.6     5     5                                              CoSO.sub.4 :0.01                                                      Com-    NiCl.sub.2 :0.01                                                                               6.5     3     1                                      parison                                                                       Example 5                                                                     Com-    Co(NO.sub.3).sub.2 :0.03                                                                      27.4     2     4                                      parison                                                                       Example 6                                                                     Com-    Japanese Patent Appli-                                                                        (note 2) 5     1                                      parison cation Laid Open                                                                               5.2                                                  Example 7                                                                             Number 59-177381.                                                             Example 1-d. (note 1)                                                 Com-    no pretreatment --       1     5                                      parison                                                                       Example 8                                                                     ______________________________________                                         Note 1: Rinse with tap water after the cobalt treatment.                      Note 2: Reported as the cobalt addon.                                    

(5) Drying: maximum sheet temperature=60° C., drying time=3 seconds

(6) Black rust accelerated testing: Test specimens (70×150mm) were cutfrom each sample test sheet. The test surfaces of the test specimenswere faced against each other to give pairs. 5-10 pairs were stacked onone another and then wrapped with vinyl-coated paper. The four cornerswere bolted down and a load of 70 kgf was applied using a torque wrench.The assembly was then maintained in a humidistat at 49° C. and 98 % RHfor 240 hours. After removal, blackening of the overlaid areas wasvisually evaluated according to the following evaluation scale:

5: no blackening

4: extremely light greying

3: blackening over <25%

2: blackening over 25 through <50%

1: blackening over at least 50%

(7) White rust accelerated testing: A test specimen (70×150 mm) was cutfrom each sample test sheet. This test specimen was subjected to thesalt-spray test specified in JIS-Z 2371 for 120 hours. The area overwhich white rust developed was then visually evaluated and reportedaccording to the following evaluation scale:

5: no white rust

4: area of white rust development <5%

3: area of white rust development=5 to <25%

2: area of white rust development=25 to <50%

1: area of white rust development at least 50% Examples 13-18 andComparison Examples 9-12

The sample test sheet (specified below) was subjected to pretreatmentaccording to the present invention or a comparison method. The sampletest sheet (both pretreated and non-pretreated) was then subjected to achromate treatment. The results of bending testing (see below) andcorrosion testing on the painted sheet are-reported in Table 3 for thesample test sheets which had been subjected to these treatments.

(1) Sample test sheet: ultralow lead hot-dip-galvanized steel sheet(Pb=0.003%), nonoiled zinc add-on=90 g/m²

(2) Pretreatment: immersion in the aqueous solution as reported in Table3, then roll squeegee to a wet pick-up of 5 mL/m²

(3) Drying: maximum sheet temperature=40° C., drying time=7 seconds

(4) Chromate treatment: A dispersion of SiO₂ in a partially reducedchromic anhydride solution (chromium ratio Cr³⁺ /Cr⁶⁺ =0.5) was used. Itcontained Si₀₂ at a CrO₃ : SiO₂ =1:1 weight ratio based on the totalquantity of chromic acid (as CrO₃) before reduction. It was applied byroll coating to give a chromium add-on of 55-60 mg/m².

(5) Drying: maximum sheet temperature=100° C., drying time=10 seconds

(6) Painting: back-surface alkyd paint paint film thickness: 6micrometers baking conditions: maximum sheet temperature=210° C. dryingtime=20 seconds

(7) Bending test: Each sample test sheet was subjected to 2T/tapepeeling and the extent of peeling was then evaluated and

                  TABLE 3                                                         ______________________________________                                                                            corrosion                                                                     resistance                                                                    testing                                   treatment solution film     bend-   on the                                    composition        weight   ing     painted                                   (mole/L)           (mg/m.sup.2)                                                                           test    sheet                                     ______________________________________                                        Example                                                                              NiSO.sub.4 :0.10                                                                              77.4     5     no abnor -13    mality                  Example                                                                              CoSO.sub.4 :0.05                                                                              38.7     5     no abnor-                               14                                    mality                                  Example                                                                              Ni(H.sub.2 PO.sub.4).sub.2 :0.04                                                              50.5     5     no abnor-                               15                                    mality                                  Example                                                                              CoSO.sub.4 :0.07                                                                              92.1     5     no abnor-                               16     Ni(H.sub.2 PO.sub.4).sub.2 :0.03                                                                             mality                                  Example                                                                              Co(H.sub.2 PO.sub.4).sub.2 :0.02                                                              25.3     4     no abnor-                               17     Na heptogluconate:0.01         mality                                  Example                                                                              NiSO.sub.4 :0.01                                                                              15.5     4     no abnor-                               18     CoSO.sub.4 :0.01               mality                                  Com-   NiCl.sub.2 :0.05                                                                              32.4     4     8D                                      parison                                                                       Ex. 9                                                                         Com-   Co(NO.sub.3).sub.2 :0.02                                                                      18.3     2     no abnor-                               parison                               mality                                  Ex. 10                                                                        Com-   Japanese Patent Appli-                                                                        (note 2) 4     8M                                      parison                                                                              cation Laid Open                                                                               8.8                                                   Ex. 11 Number 59-177381.                                                             Example 1-d. (note 1)                                                  Com-   no pretreatment --       1     no abnor-                               parison                               mality                                  Ex. 12                                                                        ______________________________________                                         Note 1: Rinse with tap water after the cobalt treatment.                      Note 2: Reported as the cobalt addon.                                    

reported according to the following evaluation scale:

5: no peeling

4: cracking only or peeled area <5%

3: peeled area=5 to <25%

2: peeled area=25 to <50%

1: peeled area at least 50%

(8) Corrosion resistance testing on the painted sheet: A test specimen(70×150mm) was cut from each sample test sheet and then subjected to thesalt-spray test specified in JIS-Z 2371 for 360 hours. The blistersproduced on the painted surface of the sheet were evaluated inaccordance with the directions from ASTM (American Society for Testingand Materials).

Benefits of the Invention

As the results in Tables 1 through 3 make clear, the results from eitherblack rust accelerated testing or white rust accelerated testing werepoor in each of Comparison Examples 1-8. For Comparison Examples 9-12,the results for either bending testing or corrosion resistance testingon the painted sheet were poor. On the other hand, Examples 1-12according to the present invention afforded excellent results for bothblack rust accelerated testing and white rust accelerated testing, andExamples 13-18 gave excellent results for both bending testing andcorrosion resistance testing on the painted sheets. Thus, through theexecution of a chromating treatment on a zinc or zinc alloy surfaceafter it has been pretreated according to the method of the presentinvention, one obtains the industrially useful effect of a well balancedincrease in the blackening resistance, corrosion resistance,post-painting corrosion resistance, and paint adherence.

We claim:
 1. A process for forming a protective coating on a zincsurface, comprising a step of applying a chromate treatment to the zincsurface, wherein, prior to applying the chromate treatment to the zincsurface, the zinc surface is pretreated by a process comprising stepsof:(A) covering the zinc surface with a layer of an aqueous solutionthat consists of (i) water, (ii) at least one Ni²⁺ or Co²⁺ salt, withone of the anions sulfate, PO₄ ³⁻, HPO₄ ²⁻, H₂ PO₄ ⁻, P₂ O₇ ⁻², HP₂ O₇⁻, PO₃ ⁻, and mixtures thereof and, optionally, (iii) a complexingagent; and, without any intervening water rinse, (B) drying into placeon the zinc surface covered in step (A) the solids content of the layerof aqueous solution applied in step (A) by removing the water therefromat a temperature within the range of 40°-100° C.
 2. A process accordingto claim 1, wherein the layer of aqueous solution applied to the zincsurface in step (A) contains from 0.5 to 100 mg/m² of the zinc surfaceof salts selected from the group consisting of sulfate and phosphatesalts of Ni²⁺ and Co²⁺ and mixtures thereof.
 3. A process according toclaim 2, wherein the aqueous solution applied to the zinc surface instep (A) additionally comprises a complexing agent selected from thegroup consisting of gluconic and heptogluconic acids and their salts. 4.A process according to claim 1, wherein the aqueous solution applied tothe zinc surface in step (A) additionally comprises a complexing agentselected from the group consisting of gluconic and heptogluconic acidsand their salts.
 5. A process according to claim 4, wherein the coveringof step (A) is accomplished by roll coating.
 6. A process according toclaim 3, wherein the covering of step (A) is accomplished by rollcoating.
 7. A process according to claim 2, wherein the covering of step(A) is accomplished by roll coating.
 8. A process according to claim 1,wherein the covering of step (A) is accomplishes by roll coating.